A mathematical function describing the geometry of spacetime in a model which incorporates the cosmological principle. In general, a metric relates physical distances or intervals between events separated in space and/or time to the coordinates used to describe their position. General relativity deals with a four-dimensional spacetime in which the separation between space and time coordinates is not obvious. In a homogeneous and isotropic cosmology, however, it is possible to define a unique time coordinate, called cosmic time, and three spatial coordinates. The Robertson–Walker metric is the most general possible four-dimensional metric function compatible with homogeneity and isotropy. In general, it describes a curved space which is either expanding or contracting with cosmic time. It is named after the American mathematician and cosmologist Howard Percy Robertson (1903–61) and the English mathematician Arthur Geoffrey Walker (1909–2001).
Subjects: Astronomy and Astrophysics.